Low Coefficient Of Thermal Expansion Materials Research Articles

Materials Selection in Gas Turbine Engine Design and the Role of Low Thermal Expansion Materials

Materials selection criteria in gas turbine engine design are reviewed, and several design challenges are introduced where selection of low coefficient of thermal expansion (CTE) materials can help improve engine performance and operability. This is followed by a review of the types of low CTE materials that are suitable for gas turbine engine applications, and discussion of their advantages and disadvantages. The primary limitation of low CTE materials is their maximum use temperature; if higher temperature materials could be developed, this could result in novel turbine system designs for gas turbine engines.

Challenge to zero CTE and lower shrinkage organic substrate material for thin CSP package

Along with the advancement in miniaturizing of mobile devices, typified by smart phones and tablet PCs, the semiconductor PKG substrate installed in these devices is demanded to be thinner and higher in density. As one of the most innovative solutions, the PoP (package on package) technology, which has the three-dimensional construction, has been expanding rapidly in recent years. However, thinner PKG such as PoP tends to warp at the assembly process and cause the decrease in the connection reliability. Therefore ultra low CTE (coefficient of thermal expansion) core materials have been needed as a key solution for the reduction of the warpage for PoP. We have already developed and mass-produced ultra low CTE core material named E-770G (CTE value: 1.8 ppm/°C) for this application. However, the demand for lower warpage is still stronger especially for thinner PKG applications. To meet this requirement, we confirmed that not only low CTE but also lower resin shrinkage control gives big impact to warpage. In this paper, we would like to introduce new ultra low CTE and low shrinkage core material, named E-777G for next generation thin CSP. At the same time, we would like to explain the relationship between the warpage and material properties (CTE and resin shrinkage). E-777G has ultra low CTE and low shrinkage. These properties were achieved by applying our original resin system designed by hard segments and soft segments. Hard segments have a stack structure of aromatic ring and the strong intermolecular force between them, which lead to the ultra low CTE and low shrinkage. Soft segments of low elastic modulus can follow approximately the thermal behavior of glass fabric when heated and cooled. Consequently, the material can show the ultra low CTE which is similar to that of glass fabric itself. As a result, E-777G has achieved the ultra low CTE of 0.7 ppm/°C by TMA method which leads to significant reduction of the warpage. In the cooling process after the reflow, the bottom PKG of PoP shows a convex warpage. It is caused by resin shrinkage of substrate in the cooling process. As the method of decreasing resin shrinkage in the cooling process, we focused on decreasing free volume of the resin. By using molecular dynamics simulation, we designed the most suitable resin composition. Confirming the warpage property, we evaluated the warpage behavior of the bottom PKG before/after assembly process. The bottom PKG using E-777G material showed lower warpage than E-770G. We confirmed that not only the ultra low CTE but also low shrinkage of material effectively contributes to the decrease of the warpage.

Through Silicon via Filling by Electroplating Using Reduced Graphene Oxide (rGO) as a Conducting Layer

Graphene materials, including single layer and multi layer graphene platelet, have recently drawn extensive attention due to their outstanding electrical and thermal properties. Reduction of grapheme oxide (GO) is a method for preparing the graphene film. Since GO has a large amount of oxygen functional groups, it can be well dispersed in several solvents and enable it to be coated on a substrate by using a chemical grafting method[1].Three-dimensional (3D) chip stacking is a major focus in recent research and development of microelectronics, MEMS, and MOEMS technology. Through silicon vias (TSVs) play a key role in the 3D IC chip stacking connections. A main advantage of TSV is to make the shortest chip-to-chip vertical interconnection, which allows for size reduction of the chip and reducing signal transmission delay[2]. Electrodeposition plays an important role for TSV metallization, especially copper electrodeposition, which is a critical technology and generally used in the 3D chip packaging.For TSV technology, the thermo-mechanical fatigue may lead to failure in the TSV interconnects[3] because the coefficient of thermal expansion (CTE) of copper is much higher than that of silicon. The package materials with different CTEs will induce large stresses at interfaces. In order to overcome this problem, copper should be substituted by tungsten. However, tungsten cannot be directly plated from an aqueous electrolyte, it can be co-deposited with iron group metals. Alternatively, since the CTE of graphene is much closer to silicon than copper, we choose graphene sheets to substitute copper seed layer.Traditional process for TSV fabrication is a dry process that includes the following step: (1) formation of vias by reactive ion etching; (2) formation of a SiO2isolation layer; (3) deposition of a TiN barrier layer and a copper seed layer; (4) electrodeposition of copper inside the via. In our research, we reduce the procedure of TSV fabrication using a wet process to substitute barrier and seed layer with graphene, and electrodeposition with Ni-W alloy to make a copper-free TSV. In other words, a copper-free TSV with simplified process steps is fabricated, leading to a low fabrication cost. Moreover, a low-stress TSV filled with low CTE materials (Ni-W and rGO) is also fabricated to improve the thermo-mechanical fatigue of TSV, leading to better package reliability.

우주용 구조 재료의 초정밀 열팽창계수 측정시스템 설계

우주용 구조물에 사용되는 재료는 치수 안정성을 위해서 열팽창계수를 최소화하도록 설계, 제작되어야 한다. 이를 위하여 우주용 재료 시편의 정밀한 열팽창계수를 측정하여, 그 재료의 열 특성에 대한 정확한 데이터를 확보할 필요가 있다. 그리고 시편뿐만 아니라 실제 사용될 구조물의 열 변형을 우주 환경에서 직접 측정할 필요가 있다. 따라서 본 연구에서는 고진공에서 정밀한 변위 측정을 위하여 변위 측정 간섭계와 항우연의 보조 챔버를 활용한 열팽창계수 측정 시스템을 설계하였다. 또한 본 측정 시스템은 길이 500mm 정도의 긴 구조물의 열 변형 Structures being used in space environment, should be designed to have minimum CTE(coefficient of thermal expansion) for the dimensional stability. Accurate CTE data of the materials are required to design the space structures consisting of various materials. There are uncertainties in the characteristics of materials even though the same manufacturing processes are applied. Therefore, it is needed to measure the thermal deformation of not only the material specimen but also substructures in simulated space environment, such as high vacuum condition. In this research, therefore, precise CTE measurement system using displacement measuring interferometer and vacuum chamber has been designed with uncertainty analysis of the measurements. This system can be used to measure the CTE of the specimen or thermal expansion of the substructure with varying size up to 50cm in length. To measure the low CTE material, overall uncertainty of this system is expected under 0.01ppm/K.

1724 CFRP/金属アクティブラミネートの作製と特性評価(J08-6 材料・構造,デバイス(2),J08 知的材料・構造システム)

This paper describes development of high performance CFRP (Carbon Fiber Reinforced Plastics) metal active laminates which can work as actuating panels only bending in the fiber direction driven by electric resistance heating of the carbon fiber. The development was undertaken mainly by investigating the effect of kind and thickness of the metal layer on their deformation characteristics. Various types of the active laminates were made by hot-pressing of a metal layer as a high CTE (Coefficient of Thermal Expansion) material, a unidirectional CFRP prepreg as a low CTE and electric resistance heating material, and a unidirectional KFRP (Kevlar Fiber Reinforced Plastics) prepreg as a low CTE and insulating material. Performance of the active laminate under constant thicknesses of the CFRP and KFRP prepregs kept at mono-layer were examined and it was found that the pure aluminum and the magnesium alloy realized high level and unidirectional actuations, and the stainless steel realized a actuation a little less than these levels and unidirectional

Proposal of an active composite with embedded sensor

Though advanced composites with embedded actuator materials such as shape memory alloys and piezo ceramics have been developed as active materials, another one by making use of thermal deformation of composites was proposed and an active laminate was prepared as an example by hot-pressing of aluminum plate as material of high coefficient of thermal expansion (CTE), uni-directional carbon fiber reinforced plastics (CFRP) prepreg as low CTE material and electric resistance heater, polymer adhesive film as insulator between them, and copper foils as electrodes. Actuation of this laminate is different from that of bimetal because CTE of the CFRP layer is strongly anisotropic due to directionality of its reinforcement fiber. As CTEs of the CFRP layer and the aluminum plate in the fiber direction are quite different from each other though they are close to each other in the transverse direction, smooth and uni-directional actuation becomes possible. In this study, its fundamental performances such as shape change and output force were observed and evaluated, and after establishment of its fabrication, an optical loss type sensor was formed in the active composite, by embedding multiply pre-notched optical fiber in the CFRP layer and breaking it at the pre-notches under bending, followed by lamination on aluminum plate with adhesive. As the sensing part can be formed inside the matrix without any complicated processes, a robust and low cost sensor is obtained. From the results, it becomes clear that: (1) curvature of the active composite linearly changes as a function of temperature between room temperature and its hot pressing temperature by electric resistance heating of the CFRP layer and cooling, (2) its output force against a fixed punch during heating from room temperature up to around glass transition temperature of the resin phase almost linearly increases with increasing temperature, (3) the multiply pre-notched, embedded and fractured optical fiber works as a sensitive sensor for monitoring the curvature of the active composite.

Tailored Coefficient of Thermal Expansion Printed Wiring Boards to Improve the Solder Joint Life of Leadless Ceramic Chip Carriers

A computer program was used to predict the coefficient of thermal expansion (CTE) of four printed wiring board (PWB) designs. Designs consisted of low CTE materials, E glass/epoxy and copper. Twenty PWBs were fabricated to duplicate four computer models. CTE was measured by a strain gauge technique developed at Boeing. The measured CTE was then compared with the predicted CTE value and a design curve developed. Following CTE verification, the PWBs were assembled with 20, 44, 68, and 84 I/O leadless ceramic chip carriers. The printed wiring assemblies were then thermal shocked from −55 to 125°C and continuously monitored to detect the first solder failure for each I/O chip carrier. The results illustrate the dependence of solder joint life on PWB CTE and verify the use of a computer model to design surface mount PWBs.